Although all birds rely on internal fertilization for reproduction, the males of just 3% of bird species actually possess penises —instead, males and females alike invert and bring together their cloacas, a basic opening through which all the birds' various bodily emissions pass.

That's particularly strange, because male birds do have the genetic coding necessary for a penis, and they do briefly have genitalia as embryos. But during their development, a genetic programs of sorts is triggered that cuts off the cell growth for the penis, and by the time the males are born, only a rudimentary vestige is left over. According to researchers from the University of Florida, the key gene here is known as Bmp4, which is active in species like chickens that do undergo this transformation but not found in the small number of bird species, such as ducks and emus, that retain normal penises. In a statement, team member Martin Cohn discusses the finding:

"Regulation of the balance between cell proliferation and cell death is essential for controlled growth and development. Too much cell division or too little cell death can lead to overgrowth or mis-regulated growth, as in cancer. If the balance is tipped in the other direction, deficient cell division or excess cell death can lead to underdevelopment or even absence of a tissue or organ. Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell death in a new location, the tip of the emerging penis.

"Genitalia are one of the fastest-evolving organs in animals, from mollusks to mammals. It is also the case that genitalia are affected by birth defects more than almost any other organ. Dissecting the molecular basis of the naturally occurring variation generated by evolution can lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected. This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations."

But just why do the males of so many bird species lose their penises? There's no clear evolutionary advantage for this, but the researchers speculate that this provides the females far more choice when it comes to reproductive partners, and that in turn presumably makes for more successful species overall. However, that's only a preliminary explanation—for now, the simple fact that birds have genetic programs that shrivel away their penises seems like more than enough news for one day.